How to apply a texture during an animation?
You can animate the 'Fac' property of a 'MixRGB' node to switch between two different textures, one not burnt and one burnt:


Connect your unburnt texture to the top input of a 'MixRGB' node (Shift+A Colour> MixRGB).
Connect your burnt texture to the bottom input.
Go to the frame just before where you want the burn to appear.
Set the 'Fac' value of the 'MixRGB' node to '0'.
Press I when your cursor is over the 'Fac' value of the node to insert a keyframe.
Go to the next frame, change the 'Fac' value to '1' and insert another keyframe.




For a smoother fade between the textures simply insert the second keyframe a few frames after the first, the more frames, the slower the fade between the two textures.